1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image (electrostatically charged image), used in an image forming process such as electrophotography, electrostatic recording or magnetic recording, and a process for producing such a toner. The present invention also relates to a binder resin and a process for producing it.
2. Related Background Art
Methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth are conventionally known as electrophotography. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member, utilizing a photoconductive material and according to various means, subsequently developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor. In the case when the process comprises a toner-image transfer step, the process is usually provided with the step of removing the toner remaining on a photosensitive member.
As developing processes in which an electrostatic latent image is formed into a visible image by the use of a toner, known methods include the magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, the cascade development as disclosed in U.S. Pat. No. 2,618,552, the powder cloud development as disclosed in U.S. Pat. No. 2,221,776, and the method in which a conductive magnetic toner is used, as disclosed in U.S. Pat. No. 3,909,258.
As toners used in these development processes, fine powder obtained by dispersing a dye and/or pigment in a natural or synthetic resin has been hitherto used. For example, particles formed by finely grinding a binder resin such as polystyrene comprising a colorant dispersed therein, to have a size of about 1 to 30 .mu. are used as the toner. A toner incorporated with magnetic material particles such as magnetite is also used as the magnetic toner. On the other hand, in a system in which a two-component type developer is used, the toner is used as a mixture with carrier particles such as glass beads, ion powder and ferrite particles.
Nowadays, such recording processes have been widely utilized not only in commonly available copying machines, but also in printers for output information from a computer, or for the printing of microfilms (or copying from microfilms). Accordingly, a higher performance has become required, and the above recording processes have now been required to simultaneously achieve the improvements in performance such that an apparatus is made small-sized, lightweight, low-energy, high-speed, maintenance-free, and personal. In order to meet these requirements, the needs on toners have become severe in various aspects.
For example, with wide application of the recording processes and wide use thereof in offices or homes as mentioned above, it has become necessary of course for toners to use safe materials and also for manufacturers to pay attention to odors therefrom that are generated in the course of fixing.
For example, with wide application of the recording processes such as electrophotography and wide use thereof in offices or homes as mentioned above, it has become necessary, of course, for developers to use safe materials and besides for manufacturers to take care of odors generated in the course of fixing. In most instances, deodorizing filters for absorbing odors or ozone are attached to copying machines, printers and so forth. This is not only disadvantageous for production cost, but also troublesome for the maintenance of deodorizing power because of periodical replacement of filters.
With regard to this problem of odors, this is of course an important problem when the viewpoint of users or manufacturers is taken into consideration. In general, it is by no means easy from the technical side to solve this problem, when viewed from a fixing method in which a toner containing a synthetic resin as a main component is fixed on a transfer medium such as papaer by utilizing the action of softening and melting by heat. It is not advisable to lower, for example, development performances such as image quality and durability of toner in order to solve this problem.
Various methods have been proposed so that these performances can be satisfied at the same time. However, a method in which an additive is used may often cause unexpected problems.
For this reason, it is preferred to improve the binder resin itself. For the purpose of such an improvement, it is considered better to decrease the amount of a residual solvent or residual polymerizable monomers that produce odors. Various methods have been hitherto proposed with concurrent attention to the influence on other development performances.
For example, Japanese Patent Application Laid-Open No. 55-155632 proposes that a polymer in which the content of a solvent used for obtaining the polymer or that of polymerizable monomers is less than 0.1% by weight is used as a binder resin for a toner so that the offset resistance, storage stability and fluidity of the toner can be improved.
Japanese Patent Application Laid-Open No. 53-17737 also discloses that residual polymerizable monomers have influence on the triboelectricity, blocking resistance and fixing properties of a toner, and proposes to decrease a residual solvent or residual polymerizable monomers of the solvent or monomers used for obtaining a polymer as a binder resin to be used for a toner.
Japanese Patent Application Laid-Open No. 64-70765 also proposes a resin for a toner containing 200 ppm or less of residual monomers, in order to cope with the problems that a work environment becomes unsatisfactory because of the odors generated when toner materials are melted and kneaded or that odors are generated at the time of copying to give an unpleasant environment. This publication also discloses that an amount more than 200 ppm of residual monomers may result in a lowering of blocking resistance and resistance to a vinyl chloride plasticizer, leaving the problem of odors.
When, however, the problem is particularly focused on the odors, it can not be said to be sufficient to only decrease the amount of residual polymerizable monomers, taking into account the possibility of wider application in the future of the image forming method such as electrophotography and the electrostatic recording method.
In general, methods of polymerizing binder resins for toners are known to include solution polymerization, bulk polymerization and suspension polymerization.
In particular, as disclosed in Japanese Patent Application Laid-Open No. 63-223014, suspension polymerization requires no removal of solvent and also requires no strong stirring, and hence enables easy production of a resin. It also makes it possible for a resin to contain in any desired amount of a THF-insoluble component that has great influence on the fixing properties and offset resistance of a toner. Thus, this is a preferable method.
In suspension polymerization, however, the resin is obtained in the form of beads. Hence, there is no step of removing a solvent or polymerizable monomers that remain in solution polymerization in which a resin is in a molten state, and instead merely has a drying step carried out for the purpose of removing water content. As a result, solvents or polymerizable monomers tend to remain in a large quantity, and therefore it is necessary to pay particular attention to the problem of odors as discussed above.